A general protection relay is set in a substation or the like, captures a system electrical quantity, AD analog/digital)-converts analog data concerning the system electrical quantity into digital data, performs a relay operation such as accident detection based on the AD-converted data, and outputs the data to an external contact. In this way, the general protection relay performs a series of processing within the same unit.
On the other hand, a process bus-adapted protection relay includes a merging unit (MU) set near a primary apparatus of a power system and configured to capture a system electrical quantity, a unit called IED (Intelligent Electric Device) set in a place remote from the MU (e.g., a protection control room of a substation) and configured to perform a relay operation, and a communication line (referred to as “process bus”) such as an optical fiber configured to connect the MU and the IED. The process bus-adapted protection relay converts analog data into digital data in the MU. The relay operation is executed according to the digital data transmitted to the IED via the process bus.
To secure operation reliability, the conventional protection relay is configured to make the units redundant, when an abnormality occurs in any of the units, continue protection with the normal units, or divide the units into a main unit and an auxiliary unit and, when a failure occurs, switch the main unit to the auxiliary unit so as not to stop the protection. For example, the related art described in Patent Literature 1 proposes that, in electrical quantity input circuits (equivalent to the MU) in which system electrical quantities are input in two systems, one of the system electrical quantities is normally input to a protection relay (equivalent to the IED) and, when an abnormality detection circuit detects an abnormality of the system electrical quantity, the electrical quantity input circuit is switched to the other normal electrical quantity input circuit so as to input the system electrical quantity.